Suspension Bridge

The suspension bridge reigns supreme for spans in excess of 600 meters. A suspension bridge is a reversed arch bridge, incorporating cables of high strength wires which have a much higher economic-resistant coefficient than that of typical steel. This economic resistant coefficient is one of the greatest advantages of the suspension bridge.

Evaluacin de puente colgante.Para la evaluacin de un puente colgante se requiere de la informacin del puente, esta se puede obtener de un archivo de inventario o de inspeccin de campo.

Introduction to assessmentThe purpose of assessment of structures is to check theiradequacy for specific loading levels and to identify thosebridges that have an unacceptable risk of failure, either inpart or complete collapse, under extreme circumstances ofloading and material condition.Many of the factors that bring about structural collapsecannot be taken into account in calculation, e.g. undiscoverablecondition, freak events. However, calculation-basedassessment is the only practical means available at presentfor gaining assurance about the adequacy of the wholestock of highway structures.The absence of any apparent signs of distress in a substandardstructure does not mean that it is structurallyadequate. When the failure mode is likely to be brittlethere may be no early warning signs. Furthermore, endrestraint or composite action which cannot be relied uponat all times in certain older structures, may temporarilyprevent such a structure from showing distress.In general, structures are assessed by the application oflimit state principles, usually at the ultimate limit state,although more modern structures are often also assessedat the serviceability limit state. The modified MEXEmethod for masonry arch bridges determines allowableloads directly, rather than in terms of limit state. Castiron bridges are assessed on a permissible stress basiswhich would exclude the risk of fatigue failure.The assessment load effects S_A are given by:_f3 effects of Q_A _f3 effects of _f1 _ Qkwhere _f3 is the factor taking account of inaccurate assessmentof the effects of loading, _f1 is the partial factorgiven in the standards for each type of loading, Qk representsthe nominal loads, and Q_A are the assessment loads.The assessment resistance R_A is given by:R_A Fc _ functionfk_mwhere Fc is a condition factor Fcm allowing for any deficienciesnoted in the inspection but which cannot be allowed forin the determination of the calculated resistance R_, fk is thecharacteristic (or nominal) strength of the material, and _mis the partial factor for material strength given in thestandards. For steel and wrought iron R_ 1=_m_function fk. For cast iron R_ function fp where fp isthe permissible stress.Structures shall be deemed capable of carrying the assessmentload when R_A5S_A, the superscript asterisk referringto the factored value.The ratio of R_A and S_A is normally tabulated in assessmentreports for each force action in each element andfor each principal loading. R_A=S_A is known as the capacityratio (CR) and S_A =R_A as the usage factor (UF). ThusCRs less than unity or UFs greater than unity are unsatisfactory.Assessment of an existing structure may be carried out, if itfails the initial assessment, in stages of increasing complexity,with the object of determining its adequacy with minimumeffort. Early stages may contain conservative means of determiningload effects. Provided that a structure is shown to beadequate at these stages, then no further analysis would berequired. However, if a structure is found to be inadequateat an early stage then assessment work should continue,and later stages should seek to remove any conservatism inthe assessment calculations. In some cases, the end resultwill quickly become self-evident and the process can beterminated at an early stage.Assessment may be carried out in five distinct levels,increasing in sophistication from Level 1 to Level 5; theseare discussed in the following section. Means for carryingout assessments at Levels 1, 2 and 3 are contained withinexisting assessment Standards and Advice Notes.Levels of assessmentLevel 1 assessmentThis is the simplest level of assessment, giving a conservativeestimate of load capacity. At this stage, only simpleanalysis methods are necessary, and full partial safetyfactors from the assessment standards are used.To facilitate such an assessment, the Highways Agencysassessment Standard BD 21 and the accompanying AdviceNote BA 16 have been developed. These documents themselvesrepresent a considerable relaxation compared todesign standards, which would be the only alternative intheir absence. It should be recognised that the use ofdesign standards for assessment would result in structuresfailing assessment unnecessarily.In addition to BD 21, assessment versions of the designstandards for concrete, steel and composite bridges, i.e.BD 44, BD 56 and BD 61 respectively, have also been developedby the Highways Agency for the purpose of providingmore realistic assessment for these bridge types.Level 2 assessmentThis next level of assessment involves the use of morerefined analysis and better structural idealisation asindicated by the existing Standards and Advice Notes.More refined analysis may include grillage analyses orpossibly finite-element (FE) analyses whenever it is consideredthat these may justify higher capacities. Non-linearand plastic methods of analysis (e.g. yield line or orthotropicgrillages) may also be used.This level also includes the determination of characteristicstrengths for materials based on existing available data.This may be in the form of existing mill test certificates orrecent tests on another similar structure.

Level 3 assessmentLevel 3 assessment includes the option to use bridge specificassessment live loading (BSALL). For long-span bridges,where the 40 t assessment fails by a small margin andwhere the bridge is located on a lightly trafficked road,then use of BSALL may be beneficial.Level 3 assessment may make use of both material testingto determine characteristic strength or yield stress and alsoworst credible strength (see BD/BA 44) or worst credibleyield stress (see BD 56 (Appendix H)).Level 4 assessmentLevel 1 to 3 assessments are based on code implicit levels ofsafety, incorporated in the nominal values of loads andresistance parameters and the corresponding partial safetyfactors. The corresponding reliability is related by implicationto past satisfactory performance of the bridge stockthrough calibrations, where these have been carried out,which involve an element of averaging.The resulting rules may however be overconservative fora particular structure. Level 4 assessments can take accountof any additional safety characteristic in that structurethrough rigorous reliability analysis, or by judgementalchanges to the partial safety factors. See HighwaysAgency Standard BA 79.Level 4 assessment may be beneficial in the followingcircumstances:n The bridge assessment criteria have been primarily devised forlongitudinal effects on main deck members. All other elementssuch as cantilever slabs, cross-beams, pierheads etc. may beexamined in Level 4 for determining element-specific targetreliability.n The whole-life reliability of a structure, in the absence of anysignificant deterioration, increases from the day it is constructedto the end of its functional life. This effect has notbeen taken into account in the present criteria.n A bridge over a very small watercourse is inherently safer thanthe average bridge, because of its much lower consequence offailure. Such considerations may be used in a Level 4 assessment.The background knowledge and engineering judgementrequired for this level of assessment is of a high order.Level 5 assessmentLevel 5 assessment involves reliability analysis of particularstructures or types of structure. Such analyses require probabilitydata for all the variables defined in the loading andresistance equations. The techniques for determining theprobability of failure from such data are now availableand can be undertaken relatively easily in modest timeframes.Level 5 assessment provides greater flexibility but itshould be noted that the results are very sensitive to the statistical parameters and the methods of structural analysis used. At present therefore Level 5 assessment should notbe used in conjunction with a prescribed target reliability,as there is no guarantee of achieving consistency in differentassessments. However it is considered by the HighwaysAgency that Level 5 may be used if the target reliability isdetermined specifically by the same assessing organisationfor a class of identical structures or structural elements(e.g. pier cross-heads), taking the reliability of the structuresas designed in respect of the assessment load as the targetreliability.Level 5 assessments require specialist knowledge andexpertise and are only likely to be worthwhile and possiblein exceptional cases.

Steel structuresIn steel structures, structural movement or failure will probablyfirst become evident as movement at connectionswhere there may be material or loading defects.Cracks are potential causes of complete fracture andusually occur at connections, changes in section and otherfatigue-susceptible details, particularly when changes areabrupt. When a crack is identified all other similar detailsshould be examined.Failure of shear connectors may be indicated by separationbetween the top flange and the concrete slab, so thatinterface should be closely examined.Deformation or distortion of members may reduce theload capacity, and sighting along flanges should be carriedout, taking maximum deformation measurements whererequired.Internal surfaces of hollow sections may need to beexamined. Otherwise the comments above for concretehollow sections apply.Breakdown of protective systems is probably the mostcommon defect and it is beneficial to detect early stagesto reduce the maintenance required. Leakage of water onto horizontal surfaces or connections, where it may pond,is a common cause of defects in protective systems. Connectionsmay move or slip, causing the protective system to failby cracking. Where defects in the protection system cannotbe identified, specialist paint inspectors may be required toprovide further advice.Corrosion is usually associated with the breakdown ofprotective systems. It is important to assess the magnitude,location and form of corrosion and, if possible, identifyits cause. Inspectors should assess and record any loss ofstructural section.